style.css

html {
    box-sizing: border-box;
    background: #ffc600;
    font-family: "helvetica neue";
    font-size: 20px;
    font-weight: 200;
}

body {
    margin: 0;
}

*,
*:before,
*:after {
    box-sizing: inherit;
}

.panels {
    min-height: 100vh;
    overflow: hidden;
    display: flex;
}

.panel {
    background: #6b0f9c;
    box-shadow: inset 0 0 0 5px rgba(255, 255, 255, 0.1);
    color: white;
    text-align: center;
    align-items: center;
    /* Safari transitionend event.propertyName === flex */
    /* Chrome + FF transitionend event.propertyName === flex-grow */
    transition: font-size 0.7s cubic-bezier(0.61, -0.19, 0.7, -0.11),
        flex 0.7s cubic-bezier(0.61, -0.19, 0.7, -0.11), background 0.2s;
    font-size: 20px;
    background-size: cover;
    background-position: center;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: flex;

    flex: 1;
}

.panel1 {
    background-image: url(https://source.unsplash.com/gYl-UtwNg_I/1500x1500);
}
.panel2 {
    background-image: url(https://source.unsplash.com/rFKUFzjPYiQ/1500x1500);
}
.panel3 {
    background-image: url(https://images.unsplash.com/photo-1465188162913-8fb5709d6d57?ixlib=rb-0.3.5&q=80&fm=jpg&crop=faces&cs=tinysrgb&w=1500&h=1500&fit=crop&s=967e8a713a4e395260793fc8c802901d);
}
.panel4 {
    background-image: url(https://source.unsplash.com/ITjiVXcwVng/1500x1500);
}
.panel5 {
    background-image: url(https://source.unsplash.com/3MNzGlQM7qs/1500x1500);
}

/* Flex Children */
.panel > * {
    display: flex;
    justify-content: center;
    align-items: center;
    flex: 1 0 auto;
    margin: 0;
    width: 100%;
    transition: transform 0.5s;
}

.panel > *:first-child {
    transform: translateY(-100%);
}

.panel.open-active > *:first-child {
    transform: translateY(0);
}

.panel > *:last-child {
    transform: translateY(100%);
}

.panel.open-active > *:last-child {
    transform: translateY(0);
}

.panel p {
    text-transform: uppercase;
    font-family: "Amatic SC", cursive;
    text-shadow: 0 0 4px rgba(0, 0, 0, 0.72), 0 0 14px rgba(0, 0, 0, 0.45);
    font-size: 2em;
}

.panel p:nth-child(2) {
    font-size: 4em;
}

.panel.open {
    flex: 5;
    font-size: 40px;
}